Waveguide feed with septum for directional antenna



P. J. ALLEN 3,109,996

WAVEGUIDE FEED WITH SEPTUM FOR DIRECTIONAL ANTENNA Nov. 5, 1963 Filed April 29, 1960 PHILIP J. ALLEN United States Patent Q messes wavaourna Faun Wl'lH sue orvi DERECTEQNAL Antenna Philip .l. Allen, Nor h Forestville, Md, assignor to the United States of America as r presented by the decre- The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates in general to antenna systems and in particular to antenna feed systems for radar apparatus and the like wherein angle sensing is obtained by a horn radiator system.

Object locator systems such as radar devices and the like normally provide angle sensing whereby the direction of a distant energy return object may be determined in at least one and usually two planes, that is elevation and azimuth. Such angle sensing apparatus may be com paratively simple where only rough measurements are to be made, but in applications where accurate angular measurement capabilities are required as in gunfire control systems, missile guidance systems, and the like, complex equipment is normally required. Such equipment in addition to being complex must also be of an extremely precise nature requiring considerable accuracy and ex pcnse in construction and maintenance to secure and maintain accuracy of operation. Although sequential lobing systems as well as conical scanning systems possess certain inherent simplicity, systems of the simultaneous lobing type are of considerable importance because of the potentially greater accuracy of such systems as well as a greater degree of freedom from interference. Simultaneous lobing systems frequently employ some form of secondary aperture device such as a lens or a parabolic reflector either of which may be illuminated from a primary radiator such as a plurality of dipoles or horns placed at the focus of the secondary aperture. A difficulty with prior art systems lies in the necessity for the placement of four primary radiators each as near as possible to the focus of the secondary radiator so that energy received from four slightly divergent lobes combines in such a way as to provide angular information relative to the direction of receipt of energy from a distant energy return object. Each horn has its minimum dimensions determined by the wavelength of the signal employed so that a four horn cluster is necessarily bulky in terms of wavelength and when used in connection with a parabolic reflector provides excessive shadow of the reflector. An additional disadvantage of conventional forms of four horn cluster feed is the inherently narrow frequency band of operation resulting from the necessity for combination in phase-opposition of signals from opposing zones of reception. Phase-opposition combination is normal y accomplished in such systems by employing signal paths which differ in length by an appropriate multiple or" half wavelengths, making such feed systems sensitive to the wavelength of operation. Further difiiculties inherent in such a four horn cluster are concerned with the usual desire for providing optimum illumination of the secondary aperture by a four horn cluster to insure a minimum loss of energy around the sides of the secondary aperture and substantially uniform illumination of the aperture itself. In any event, illumination is a compromise matter which is difiicult to optimize with even a single horn much less with a four horn cluster.

It is accordingly an object of the present invention to Patented Nov. 5, 15563 2 provide a radio frequency primary radiator sysem of small size.

It is another object of the present invention to provide a single horn primary radiator which can obtain direc tion sensing information in two planes.

Another object of the present invention is to provide a primary radiator for a simultaneous lobing system by of which the overall problems of antenna beamwidth and illumination of the secondary radiator are of less consequence than in four horn cluster antennas.

Another obiect of the present invention is to provide a primary radiator for a simultaneous lobing locator systen: in which the radiator itself as well as the interconnections thereto have a broad frequency bandwith.

Another object of the present invention is to provide an antenna system for an object locator system which has symmetrical characteristics to eliminate boresight shifts as a result of changes of frequency of operation.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same ecornes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows an overall view of one embodiment of the features of the present invention.

FIG. 2 shows a second embodiment of the teachings of the present invention.

In accordance with the teachings of the present invention, a primary radiator system for a radio frequency energy operative object locator system is provided Wherein a single horn is employed to derive angle sensing information of distant objects in distinct contrast to the multi-horn arran ernents required by the prior art. The single horn is divided by a septum in one plane, typically the azimuth plane, and has a basic inside Width of the order of a wavelength in that plane. Thickness of the major portion of the horn is determined primarily by the conventional voltage breakdown considerations With in the horn and is not necessarily determined on the basis of higher order propagational modes in the Waveguide. Thus the basic horn is of a configuration that can be quite thin so as to reduce shadowing of a secondary aperture such as a parabolic reflector cooperative with the horn to produce a desired antenna pattern. The foregoing arrangement thus may be characterized as utilizing higher order modes in the horn for direction sensing purposes in one plane and employment out of phase signals for direction sensing purposes in the other plane. in any event the entire area of the horn is employed for illumination of the secondary aperture or for direct radiation as the case may be rather than requiring a plurality of horns or portions thereof each operative for separate slightly divergent sectors of space. It is to be understood that the term horn has been used in the foregoing in a generic sense, Whereas in actuality what is referred to is a feed system having a member which is basically a waveguide structure on the end of which is placed a flaring portion to provide for example, desirable impedance transition characteristics and directivity. The exact arrangement of typical apparatus and the various considerations entering into such a typical apparatus will be discussed at considerable length in connection with subsequent portions of the specification.

With reference now to FIG. 1 of the drawing, the antenna system shown therein contains a parabolic reiiector 1% which is a secondary aperture illuminated by a primary aperture, the horn device indicated in general by the numeral The horn contains a waveguide portion 12 together with a transition or flare portion 13 which provides an optimization of the impedance match between the end of the waveguide portion 12 and the secondary aperture 1% which in turn is coupled to space i lied by the numeral 16 is of the coaxial cable type.

and also an optimization of the directivity char cteristics of the system. The waveguide portion 12 contains various internal members together with various coupling ports or devices indicated by the numerals 14-, T15 and 16. As typified in the apparatus of PH}. 1 the port devices 14 and 25 are of a Waveguide nature whereas that ld6ntl- A schematic connection or the ports 1d, 15 and to a radar device 1'? is indicated, with the port 15 providing the sum signal for the apparatus, port 14 providing the azimuth difference signal, and port 1-5 providing its elevation difference signal. Radar systems employing these basic signals are now well-known in the art, it being understood or" course that the signals reter more to the receiving phase of operation than to the'transrnitting phase of operation, however in view of tae fact that the antenna system of FIG. 1 is reciprocal in nature it is obvious that with appropriate connection to a source of radio frequency energy it can be used for transmission as well as reception to which end the radar system 17 would normally include appropriate transmit-receive switching devices and other well known components of conventional radar system -he waveguide portion 12 is approximately a wave length in the broad dimension and has a narrow dhnension such as that employed for ordianry waveguides at the frequencies involved. Such thickness is normally chosen to suppress higher mode energy propagation in the waveguide and yet provide sufficient spacing betwee the sides so as to avoid voltage breakdown due to the radio frequency voltages existing in the waveguide during transmission. In any event of course, it is desired to have this dimension small so that the overall assembly presents as small as possible an obstruction to the path of energy to space. The space coupling end of the waveguide portion 12 has disposed therein a septum or partition 18 which is placed in the long transverse dimension of the waveguide being connected at the ends thereof to the narrow walls of the waveguide. Thus the open end of the waveguide 11 is divided into upper and lower portions which would be excited out of phase to a greater or less extent depending upon the direction of impinge ment of incoming energy. Where'the incoming energy impinges upon the end of the waveguide substantially head-on, the two portions of the waveguide produced by the dividing septum are energized substantially in-phase so that for all practical purposes the septum is of no consequence in the waveguide for such a condition of reception. On the other hand, however, Where energy is received either above or below the plane of the septum 38, the two halves, upper and lower, are energized with a difference in amplitude or phase so that a difference exists between the energy present above the septum and that present below the septum. This differential energy travels through the waveguide; however, it becomes transformed to a coaxial or strip line mode being propagated through the extension of the septum l8 identified by the numeral 19 where it is delivered to the elevation diiference output port is? for use by the radar system 17 in more or less conventional manner. Thus the apparatus is seen to be capable of providing an elevation differonce signal in dependency upon the angle of arrival of the energy relative to the plane of the opening of the horn assembly or the amplitude difference above and below the septum. Normally the septum 13 extends to the limits of the transition portion 13. In some instances it may be desired that this partition member extend either beyond the opening provided by the transition member 13 or terminate somewhat short of that opening. In general such a configuration may be determined on its merits as a matter of experimentation it not being actually a limiting factor as far as utilization of the principles of the present invention is concerned.

The double normal width of the waveguide portion 12 permits the propagation of the TE mode in the Waveguide when the direction of incidence oi the energy upon the Opening of the transition device 13 is other than normal to the opening thereof in the azimuth plane. When the direction of incidence is normal, the T5 mode is not excited in the waveguide. When this mode is excited it propagates down the waveguide portion 12 and is extracted by the port 14 to appear as the azimuth difference signal which is delivered to the radar device 17 for use thereby. Following the port 14, the waveguide portion 12. need no longer remain at the double width as shown is reduced to the normal half-wavelength width dime sion of a conventional waveguide in the region identified by the numeral 20. Actually the stepped portion of the waveguide in the'plane of the broad dimension provides for improved impedance match of the T5 mode to the output port 14 to avoid the effects of re lections in the overall system.

As to the basic T5 mode in the waveguide as excited by he co nponent of the incident energy which is normal to the pane of the opening 113, this energy propagates through the waveguide portion 12 and also through the reduced portion beyond the typified point 2i! and is delivered through output port 15 to the radar system 17 for use in conventional fashion as the sum signal;

Thus in summary of the previous discussion relative to El-G. 1, it is seen that the apparatus thereof provides the basic sum signal as well as the azimuth and elevation difference signals, the latter two being in separate ports as required by the conventional form of simultaneous lobing radar system which could be employed in block 17. The elevation diiference signal as obtained at port 36 is proportional to the angle of incidence of the incoring energy relative to the plane of the dividing septum 18 while the azimuth difference signal is proportional to the variations in the angle of receipt of the incoming energy in the azimuth plane relative to the normal to the open end of the transition member 13.

The foregoing apparatus is straightforward in overall arrangement and it may be readily appreciatedthat it is not the only specific arrangement of apparatus that could be employed to practice the teachings of the present invention. in some instances, for example, it might be desirable to rearrange the ports 15 and 16 so that the coaxial take-on: is from one side of the wageguide portion 12, the sum port 15 extending substantially directly as an extension of the region 29 rather than being in a right angle relationship thereto. In any event, it is important to note that the derivation of sum and diflerence signals by the device of this invention does not require the thickness or" the waveguide portion 12 to be substantially greater than the thickness of a conventional waveguide operating in its conventional TE mode.

With reference now to PEG. 2 of the drawing, the apparatus snown therein includes a variation which in some instances provides simplification with regard to the extraction of the elevation difference signal at the port 5 3 as obtained by means of a probe 51 inserted into the waveguide in the region of the septum 1%. In some situations this may be desirable, however it requires a somewhat enlarged thickness of the waveguide 12 in the region of the elevation diiference port 56 to provide for the necessary thickness of the septum 13 where the coaxial connection between the port 59 and the probe 51 is made.

an arrangement provides straight-through propagation of the basic T13 mode as delivered at the output port 5?. rather than requiring a right angled delivery arrangement as in the previous apparatus of FIG. 1.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claims the invention may be racticed otherwise than as specifically described.

What is claimed is:

1. in combination, a rectangular-waveguide having a dimension of the order of a wavelength in one plane,

a septum disposed said waveguide in said plane, first coupling means for coupling to the TE mode in said Waveguide, second coupling means for coupling to the TE mode in said Waveguide, a coaxial mode coupler having central and outer conductors, and a strip line connecting the central conductor of said coaxial mode coupler to the septum.

2. In combination, a rectangular waveguide having a dimension of the order of a wavelength in one plane, a septum disposed in said waveguide in said plane, said septum extending across said Waveguide and connected to the waveguide walls perpendicular to said one plane, first coupling means for coupling to the T35 mode in said 6 waveguide, second coupling means for coupling to the TE mode in said Waveguide, a coaxial mode coupler having central and outer conductors, and a strip line connecting the central conductor of said coaxial mode coupler to the septum.

References Qited in the file of this patent UNITED STATES PATENTS 2,759,154 Smith et al Aug. 14, 1956 2,931,033 Miller Mar. 29, 1960 2,956,275 Ashby Oct. 11, 1960 2,961,659 Kuecken Nov. 22, 1960 

2. IN COMBINATION, A RECTANGULAR WAVEGUIDE HAVING A DIMENSION OF THE ORDER OF A WAVELENGTH IN ONE PLANE, A SEPTUM DISPOSED IN SAID WAVEGUIDE IN SAID PLANE, SAID SEPTUM EXTENDING ACROSS SAID WAVEGUIDE AND CONNECTED TO THE WAVEGUIDE WALLS PERPENDICULAR TO SAID ONE PLANE, FIRST COUPLING MEANS FOR COUPLING TO THE TE10 MODE IN SAID WAVEGUIDE, SECOND COUPLING MEANS FOR COUPLING TO THE TE20 MODE IN SAID WAVEGUIDE, A COAXIAL MODE COUPLER HAVING CENTRAL AND OUTER CONDUCTORS, AND A STRIP LINE CONNECTING THE CENTRAL CONDUCTOR OF SAID COAXIAL MODE COUPLER TO THE SEPTUM. 